\section{Introduction}\label{introduction}

The ExternalInterface allows coupling EnergyPlus to the Building Controls Virtual Test Bed (BCVTB). It supports the import of Functional Mock-up Units (FMUs) for co-simulation as well as the export of EnergyPlus as a FMU for co-simulation. BCVTB is a software environment that allows expert users to couple different simulation programs for distributed simulation or for a real-time simulation that is connected to a building control system. For example, the BCVTB allows simulation of the building envelope and HVAC system in EnergyPlus and the control logic in MATLAB/Simulink, while exchanging data between the two programs as they simulate. The BCVTB can be downloaded from \url{http://simulationresearch.lbl.gov/bcvtb}. A FMU is a component which implements the Functional Mock-up Interface (FMI) standard (\url{http://www.modelisar.com}).
